1. Field of the Invention
The present invention relates to a zoom lens and an imaging device having the zoom lens, and is appropriate for an imaging device using a solid-state image pickup element, which includes a video camera, an electronic still camera, a broadcasting camera, a security camera, and so forth, and/or an imaging device including a camera using silver halide film, for example.
2. Description of the Related Art
In recent years, the imaging devices including the video camera, a digital still camera, the broadcasting camera, and the security camera that include the solid-state image pickup element, and the camera using the silver halide film, for example, have become multifunctional. Further, the entire devices have been downsized.
It is preferred that an imaging optical system used for the above-described devices be a zoom lens which is reduced in length, compact in size, and provided with a high zooming ratio (high magnification-varying ratio) and a high resolution.
Positive-lead type zoom lenses, which include a positive refractive-power lens group provided on the object side, have been known as zoom lenses satisfying the above-described demands.
A zoom lens including four lens groups of which refractive powers are positive, negative, positive, and positive in that order from the object side toward the image side has been known, as the positive-lead type zoom lens (U.S. Pat. No. 7,057,818).
A zoom lens including five lens groups of which refractive powers are positive, negative, positive, positive, and positive in that order from the object side toward the image side has been known (U.S. Pat. No. 7,286,304).
Further, a zoom lens including five lens groups of which refractive powers are positive, negative, positive, negative, and positive in that order from the object side toward the image side has been known (U.S. Pat. No. 6,594,087).
Usually, for downsizing the entire zoom lens while ensuring a high zooming ratio, the number of lenses should be decreased while increasing the refractive power of each of the lens groups included in the zoom lens.
According to the above-described zoom lens, however, as the refractive power of each lens surface increases, so does the thickness of each lens. At the same time, it becomes increasingly difficult to correct various aberrations.
For obtaining appropriate optical capabilities while attaining the high zooming ratio and downsizing the entire lens system for the above-described zoom lens including the four or five lens groups, the refractive power of each lens group and conditions under which each lens group is moved in association with zooming should be appropriately set.
Particularly, for obtaining high optical capabilities over the entire zooming range while ensuring a high zooming ratio approximately twenty times as large as the zooming ratio, conditions under which the first and second lens groups are moved in association with zooming, the refractive power of each of the first and second lens groups (the inverse of the focal length), and so forth should be appropriately set.
If the above-described configurations are inappropriately set, it becomes difficult to reduce the front size diameter, ensure a high zooming ratio, and obtain high optical capabilities throughout the entire zooming range.
For example, a moving stroke associated with zooming of a group of magnification-varying lenses becomes long, which makes it difficult to reduce the length of the lens and downsize the entire lens system.